Jeff Melcher and his pal Beau Hanson were gnawing on their burgers at the Horseshoe Bay Resort on Leech Lake, basking in the afterglow of a successful morning of walleye fishing. In a couple of hours on the water, they'd managed to land two real pigs—both around 25 inches in length—along with a mess of smaller walleyes.

"Two years ago on the opener, we came out all day and caught nothing," says Melcher, a boyish 34-year-old boat salesman from the nearby town of Walker. "It's just been a heckuva day. Really nice to see the lake rebound like this."

Just a couple of years ago, many walleye anglers on Minnesota's third-largest inland lake struggled to catch a single fish, let alone fill limits. But to the relief of both fishermen and the local tourism industry, the good times have returned. Which is why Tim Pawlenty selected the lake as the site for this year's Governor's Opener.

Indeed, most of the walleye fisheries in the state appear to be in decent health these days. Even at Upper and Lower Red Lakes, where decades of netting by tribal fishermen caused walleye numbers to crash in the 1990s, anglers are once again catching the state fish with relative ease.

Still, some of the Minnesota's leading fishery experts are worried about the walleye's future. A growing body of research suggests that climate change in the coming decades could make many lakes inhospitable to walleye.

"I don't like to sound like an alarmist, but it is certainly likely that production of walleyes is going to decline in southern Minnesota," says Don Pereira, the fisheries research and policy manager at the Department of Natural Resources. "The growing season is just going to be too warm."

Pereira, who spent much of his career studying walleye, says the culprit is carbon dioxide. If worldwide C02 emissions double, as they did between 1990 and the present, it could raise water temperatures enough to threaten walleye. A so-called cool-water fish, walleye are fairly sensitive to even incremental changes in heat. When water temperatures top 82 degrees, the fish stop feeding. At 88 degrees, they die.

A model developed by University of Minnesota researcher Heinz Steffan suggests that major changes could be in store for walleye factories such as Lake Pepin, the widening of the Mississippi River downstream of Red Wing. After some of the recent hot summers, a fisheries worker at Pepin detected evidence of "thermal stressing" among bigger walleyes, which are more sensitive to heat.

The prognosis for walleye in central and northern Minnesota is less clear. In some lakes in far northeastern Minnesota, warming trends could actually improve walleye numbers. But that would come at the expense of true coldwater fish, such as lake trout.

Over the past few years, many researchers have become convinced that climate change will alter fish dynamics in Minnesota lakes, says Pereira. "Some of our folks who have really doubted the issue are now saying, 'It really looks like there is something to climate change and it will affect our aquatic resources.'"

But not everyone is convinced. Denny Schupp, a retired DNR biologist whose expertise and research innovations earned him the nickname "Mr. Walleye," points out that most of the recent warming in Minnesota's climate has occurred in the winter nights, not summer days. Looking at summertime highs and lows on the shores of Lake Mille Lacs, Schupp found that none of the warmest summertime months had occurred in the past 25 years.

Predicting the future is generally a tricky undertaking. In the world of fisheries research, trying to understand the past—even the recent past—can be pretty confounding.

Consider the recent decline of walleye on Leech Lake. Around the time the walleye numbers were plummeting, double-crested cormorants—large, fish-eating birds that resemble a cross between a crow and a duck—had staged a remarkable recovery on Leech. Not surprisingly, anglers and many researchers were quick to blame the cormorants for the disappearance of Leech Lake walleye.

In response, the U.S. Fish and Wildlife Service, in conjunction with the Minnesota Department of Natural Resources and the Leech Lake Band of Ojibwe, set out on a campaign to kill cormorants on the lake. Over a two-year period, the number of nesting pairs on Leech dropped from 2,524 to 532.

But an examination of the stomach contents of cormorants shot in the culling has shown that walleye constitute less than 1 percent of the birds' diet on Leech Lake. Turns out they may not have been responsible for the walleye decline after all.

Despite the uncertainties of his field, Pereira thinks the angling public and resource managers ought to pay more attention to the implications of global warming.

"People need to realize that these changes are going to be happening and there are going to be some lakes that won't be suitable for certain species, like walleye, and maybe you have to learn to accept other species, like bass and panfish," he says. "The cynical side of me says that American society has become too self-absorbed and too selfish to give serious concern to the issue. Hopefully I'm incorrect."